1. Field of the Invention
This invention relates to an electrochromic display device to effect reversible coloring by the application of a voltage and a method for forming an electrochromic film to be used therefor and in particular to the WO.sub.3 -sputtered film forming process with a DC or high-frequency sputtering device and to an electrochromic display device in which high reliability and durability can be obtained by controlling the applied voltage to cause the WO.sub.3 -sputtered film to operate in the unsaturated condition.
The electrochromic display device is a non-light-emitting type display device utilizing the electrochromic phenomenon, which has a number of excellent advantages such as clear color display, smaller power consumption, no dependence on visual angle, wider operating temperature range and possibility of large-scale display, etc. in comparison with the conventional display devices such as LED and liquid crystal displays, etc. The electrochromic display device has an electrochromic film formed on a substrate, which electrochromic film is colored or discolored depending on the direction of voltage application.
2. Description of the Prior Art
Typically, an electrochromic device 1 comprises, as shown in FIG. 1, oppositely placed glass plates 3 with transparent electrode layer 2 formed thereon, and on one of the transparent electrode layers 2 is formed an electrochromic layer 4. Further, spacers 5 made of vinyl or the like are provided on the sides, in which electrolyte 6 is filled. As the electrolyte, a normal sulfuric acid with 10% by volume of glycerol mixed therein can be used, for instance. Accordingly, if a voltage is applied in a predetermined direction between the two electrodes 2, 2, the electrochromic film 4 is colored; whereas, it is discolored by the application of voltage in the opposite direction. With regard to the materials for formation of such an electrochromic film, various materials such as inorganic compounds (WO.sub.3, for instance) and organic compounds (biologen compound, for instance), etc. have been proposed and typically WO.sub.3 is employed.
Heretofore, for the formation of an electrochromic film with WO.sub.3 (tungsten oxide), WO.sub.3 -powder has been employed and the film formed on a substrate comprising glass plates coated with transparent conductive film (NESA film) by vacuum deposition process. The WO.sub.3 -film formed on a substrate under the vacuum deposition process is colored and discolored to perform the electrochromic operation on voltage impression, but industrial mass-production of such a WO.sub.3 -film having a large area is difficult and the WO.sub.3 -film has a problem in strength so that it peels off the substrate, thereby hindering practical application. To prevent such peeling, methods such as heating a substrate to high temperatures for degassing, special use of adhesives and mixing of bubble absorbent in electrolyte to suppress bubble formation in electrolyte as a cause of peeling, etc. have been proposed, but satisfactory results cannot be obtained by these conventional techniques, which in addition require special additives and cannot be said advantageous in manufacture.